In my patent application Ser. No. 221,212, now U.S. Pat. No. 4,409,841 I have described a fatigue damage indicator for a mechanical member subjected to repetitive stress, comprising two thick elements which are spaced from one another and are connected one to the other by a thin plate formed with a slot and at least one pair of free opposite edges, the two elements being so connected with the mechanical member or fixed to the latter so that they are displaceable relative to one another in a plane parallel to the plane of the thin plate.
My fatigue damage indicator is characterized in that the thick elements are disposed so as to be separated from one another in a direction substantially perpendicular to their direction of relative displacement by such stress, the thin plate being placed under shear force in its plane by such relative displacement, the slot of the thin plate extending from one of the edges of the web in the said direction and terminating short of the opposite edge in a blind end of the slot having two corners so that, with application of stress, two cracks develop at corners of the bottom of the crack and are propagated progressively in the web towards an opposite edge of the thin plate so that said cracks meet cracks propagating generally in a direction away from said opposite edge to detach or substantially detach a central zone of the thin plate delimited by said cracks.
This fatigue damage indicator is essentially intended to indicate the fatigue of a piece subjected to tensile or compressive stresses.
However, in practice, a piece is not subjected to a pure tensile or compressive force; often, it is also subjected to a shear stress or to torsional or bending moments. Such parasitic stresses and moments influence the functioning of the fatigue damage indicator and it so happens that, in practice, the web is detached for a fatigue different from that for which the indicator has been calibrated.